Given that you have your mind set on the 100-400 (my assumption?) and that you're looking to save the 30k, you seem to have your answer already...
Buying brand new lenses is generally recommended because (a) you can be assured of the condition; (b) lenses don't depreciate in value or become obsolete or get exhausted as camera bodies tend to. If you are sure that this is the lens for your needs (not just at the moment but in the long run as well), I personally would suggest getting a brand new one. However, if you do choose to buy the used one, do examine every bit of the lens carefully.

One little thing worth considering- I'm not very sure if buying a second hand lens would affect your availing Canon's services in any way- but if it does, I would recommend against it if I may.